2000 impala HVAC issue

I need some assistance with this. A funny thing happened yesterday while driving my 2000 Impala. I left the house yesterday for a 180 mile road trip had the blower on high / Defrost / high temp setting and about 20 mins into the ride I had seriously reduced air flow from the duct. I swapped to floor setting and vent settings and no airflow! I can hear the blower turning. So made it to Niagara Falls and parked the car. At noon i left my meeting and removed the plastic molding at the base of the windshield where the fresh air filter is located looked at the fan and everything seems ok, started the car and there is normal air flow. Put the car back together and started off to return home. About 20 mins in no airflow from duct work again! stopped to get gas, worked again for about 40 mins then again no flow...

I don't suspect a plugged heater core as I do get really good air flow at the beginning then it seems to slowly decline.

So the car was getting a remote start installed today and I mentioned the issue to the tech:great: doing the instal and he fixed the issue he had the dash apart checked the hoses and blew them out and all seems good!